The present invention relates to a locking mechanism for a seat belt retractor.
A seat belt retractor typically comprises a spool on which seat belt webbing is wound and which rotates about its longitudinal axis to pay out or wind in webbing as required by movement of the vehicle occupant. In the event of a crash, the spool is locked against rotation and thus pay out is prevented and the vehicle occupant is securely restrained against forward motion.
The spool is locked by engagement of ratchet teeth at its ends, with toothed pawls mounted on a lockbar extending across the width of the spool.
The lockbar is under considerable strain during a crash and must be constructed to be very strong. Typically the teeth on the locking pawls may shear under particularly high crash forces.
There is provided in accordance with the present invention a seat belt retractor comprising a retractor frame with side walls and a rotatable spool extending between said side walls, said rotatable spool having seat belt webbing wound thereon, a primary locking mechanism comprises a locking pawl for engaging with ratchet teeth on the outer periphery of the spool for arresting rotation of the spool, and a secondary locking mechanism for locking the spool against rotation.
According to a first embodiment of the invention, the secondary locking mechanism comprises a set of teeth formed in a detent that is offset from the plane of the retractor frame side walls and arranged to co-operate with a smooth surfaced spool ratchet flange to lock the spool against rotation.
According to a second embodiment of the invention the secondary locking mechanism comprises an inner frame comprising two circular ring portions and a joining portion, and a set of teeth is formed on the inside surface of each ring and arranged to engage the spool ratchet flange to lock the spool against rotation.
According to a third embodiment, secondary locking teeth are mounted on a bar fixed to the frame sidewalls across the open side of the retractor frame. This bar may be formed as an extension of the tie plate, which holds the sides of the frame together.
According to a fourth embodiment, the secondary locking teeth are mounted on a secondary locking pawl, which is pivotably mounted on a locking pawl support bar.